In recent years, lithium ion secondary batteries (hereinafter, referred to as “lithium batteries”) enabling high energy, reduced thickness, and compact size have been actively developed as secondary batteries for general use within portable terminals such as laptop computers and mobile phones, video cameras, and the like. As the packaging material used in a lithium battery, rather than metal cans which have been used as the packaging material for conventional batteries, packaging material which forms a multilayer film (such as a configuration including a heat-resistant substrate layer/an aluminum foil layer/and a thermal adhesive film layer) in the shape of a pouch has been used for the advantages of being lightweight and the degree of freedom when selecting the shape of the cell.
Lithium batteries contain, as battery contents, a positive electrode material, a negative electrode material, and either an electrolytic solution prepared by dissolving a lithium salt as an electrolyte in an aprotic solvent having a penetrative ability such as propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, or ethyl methyl carbonate, or an electrolyte layer formed from a polymer gel impregnated with the electrolytic solution. If the solvent having the penetrative ability penetrates through a thermal adhesive film layer that acts as a sealant, there are the problems that the strength of lamination between the aluminum foil layer and the thermal adhesive film layer can deteriorate, and the electrolytic solution evaporates (dries up) from the gaps between the aluminum foil layer and the thermal adhesive film layer.
Further, as the lithium salt which is the electrolyte of the battery, salts such as LiPF6 or LiBF4 may be used, but these salts generate hydrofluoric acid due to a hydrolysis reaction with moisture. Therefore, these salts can cause corrosion of metal surfaces or the deterioration of the lamination strength between each of the layers of a multilayer film. By the use of aluminum foil, moisture can be substantially blocked from penetrating through the surface of the packaging material. However, in the lithium battery packaging material, the multilayer film has a construction that is bonded by heat sealing, thus, the hydrolysis of the lithium salt due to moisture which penetrates through the edge face of the seal of the thermal adhesive film layer which functions as a seal is still a concern. Accordingly, it is important to strengthen the interlayer adhesive strength between the aluminum foil and the thermal adhesive film layer in order to have content resistance (the electrolytic solution resistance and the hydrofluoric acid resistance).
Lithium batteries are widely used in portable mobile phones, and there are cases when the use environment thereof may reach a temperature of 60 to 70° C., for example, inside a vehicle during summer, thus, it has been necessary that the lithium battery packaging material is imparted with resistance to the electrolytic solution even in high temperature environments.
Recently, the automobile industry has been developing automobiles using only secondary batteries or automobiles combining the use of gasoline and a secondary battery such as electric vehicles (EV) and hybrid electric vehicles (HEV). Further, the power storage industry has been developing electric double layer capacitors (EDLC) for storing electrical power manufactured in solar cells or wind power generators, or lithium ion capacitors (LIC) having properties of both secondary batteries and capacitors. The automobile industry and the power storage industry and the large-scale secondary battery/capacitor market have sought a battery which not only improves the performance of the battery, but has greater safety and long-term stability (10 to 30 years).
Generally, as the functions which are sought in the lithium battery packaging material used in consumer applications, inhibiting the delamination between the aluminum foil layer and the thermal adhesive film layer may be mentioned. The delamination is generated by the effects of the electrolytic solution or hydrofluoric acid generated by hydrolysis of the lithium salt which is an electrolyte.
Generally, the battery body part of the lithium battery packaging material made up of a multilayer film is mounted to the formed/drawn part formed by cold forming, and is finally sealed by heat sealing. The thermal lamination method is excellent in the point of improving the adhesiveness between the aluminum foil layer and the thermal adhesive film layer by thermocompression bonding, but when the heating is insufficient or the cooling rate is slow, crystallization progresses easily. Therefore, microcracks are easily generated in the thermal adhesive film layer due to the strain generated during cold forming, specifically, the blushing phenomenon of the film occurs easily in the drawn portions such as the side surfaces or the corners. The blushing phenomenon in cold forming lowers insulation and promotes the deterioration of the battery performance, thus, the means to not only inhibit the blushing phenomenon due to cracks, but also to inhibit the blushing due to bending has been sought.
For example, PTL 1 discloses a lithium battery packaging material. According to this patent literature, low crystalline ethylene-butene copolymer or propylene-butene copolymer, amorphous ethylene-propylene copolymer, propylene-ethylene copolymer, or the like is added to an adhesive resin to adjust the fluidity (Melt Flow Rate; MFR) of a molten plastic, so that delamination due to an electrolytic solution and hydrofluoric acid does not easily occur.
PTL 2 discloses a packaging material for batteries, which is a laminate made of a substrate layer, an adhesive layer, a chemical conversion treatment layer, aluminum, a chemical conversion treatment layer, an adhesive resin layer, and an innermost resin layer laminated in this order. In the same manner as PTL 1, an elastomer component purposing to relax stress is added to the innermost layer to impart flexibility, thereby improving bending resistance and the inhibition of blushing when formed.